Owing to the progress of magnetic recording technology, floppy disk drive units with increased recording density have become available. One example is a 51/4"-disk unit having a track density of 96 tracks per inch and a data transfer rate of 500K bits per second, providing an increased recording capacity of about 3.3 times a 51/4"-disk unit having a track density of 48 tracks per inch and a data transfer rate of 250K bits per second, and floppy disk units of different densities are often used in a mixture within a system.
These floppy disk units differ not only in the recording density, but also in the recording format so that they require respective interface controllers to connect to a data processing system. FIG. 1 shows a block diagram of the conventional data processing system having a plurality of floppy disk drive units. A host system 1 having a processor is connected via a floppy disk controller 2a operating at 250K bits/sec for 48 TPI disks, and a floppy disk controller 2b operating at 500K bits/sec for 96 TPI disks, respectively, to floppy disk drive units 3a and 3b loading floppy disks of the first recording density, i.e., 48 TPI, and the second recording desntiy, i.e., 96 TIP. Provision of such controllers for interfacing respective floppy disk units require increased floor space, and this makes the system economically unfavorable.